In the production of finger jointed lumber, it is necessary to apply glue to the finger-cut (serrated) ends of the sticks of lumber in preparation for joining the sticks end-to-end to form a longer length of lumber. The glue is generally applied while the stick is held on its side or edge, and the finger-cut end is thus vertical, and may be characterized as an uneven vertical surface. The term "glue" is used herein in a broad sense to include both protein glue and other types of adhesives which are in a flowable condition prior to curing.
It is essential to the formation of a sound lumber finger joint that the entire finger-cut end surface of at least one of the sticks making up the joint being coated with glue, and coated as uniformly as possible. At the same time, it is desired to keep glue off the sides of the lumber, and to waste as little glue as possible through spillage.
In order to impart the desired mechanical strength to a finger joint, it is the practice to finger-cut the ends fairly deeply (typically about 9/16 inch with variations encountered from about 1/8 inch to about 1 1/2 inches), making the surface of the end very uneven, and making it difficult to apply glue evenly on the surface, because the application tool must reach both the deep-cut regions of the end and the shallower portions, and deposit on them a uniform layer of glue. Furthermore, since for various operational reasons it is most common that the surface to be coated is oriented vertically, and since the glue during the application stage is a liquid, it is difficult to load the application tool with glue uniformly from top to bottom and to maintain such uniform loading, and it is thus difficult to apply a uniform layer of glue across the surface to be coated from top to bottom.